I cant keep my finger down when im playing barre chords?

i'm trying to play a song and i can play the whole thing except all the parts with barre chords. i can play any other type of chord but with barre chords i just can't keep my finger in place and pushing down hard enough. how to i practice to get rid of this problem? i know i can just play the chords over and over again but there's no point if i'm playing them over and over incorrectly